Added some optimizations:

* Generate a single BA instead of 6-instruction JUMP if possible
  (this occurs both in the creation and overwriting of the stub code)
* If possible, rewrite the ORIGINAL call to call the generated function
  directly, thus bypassing the stub entirely

Also added some statistics on how often calls are overwritten and how often the
CompilationCallback is invoked.

llvm-svn: 8376